As a writer of children's books, you are aware of the variety of art styles found in these books. To simplify matters I offer three tiers of styles to accommodate time and budget.
Acrylic Paintings: Takes the longest to execute. However, the most lush imagery can be the result. (Most costly)
Traditional w/Digital Color: Penciled-inked, then scanned for digital coloring. (Professional and pleasing results in a more timely manner. (Lowers cost and time significantly)
Colored Pencil: A personal touch that reduces time and steps even more. Colored pencils are not often used but can have surprising end results. (Least costly, least time)